Example sentences
lǎo老shī师hǎo好！
Hello, teacher!
wáng王lǎo老shī师bù不zài在。
Teacher Wang isn't here.
About this word
老 is "old" and 师 is "teacher, master". The literal reading, "old master", misleads: 老 here is a prefix of seniority and respect, not a comment on age. The teacher morpheme is 师, which also appears in 师傅 (skilled tradesman), 律师 (lawyer) and 医师 (physician). 老师 is what you call a teacher to their face, and Chinese speakers stretch it to anyone whose guidance they value, a music instructor or a senior colleague who shows them the ropes. 教师 is the formal label for the profession, the word that turns up on a form or in a job posting.
The story behind the character
老
A bent, stooped figure leaning on a cane, hair hanging loose. Xu Shen wrote 老, 考也. 七十曰老. One character, the whole back view of an old person.
师
The traditional 師 meant an army. 2,500 men made one 师. Xu Shen wrote 师, 二千五百人为师. Only later did it come to mean a teacher. So the 师 in "teacher" was once an army of 2,500.